US5250959A - Electrophotographic image forming apparatus and method - Google Patents

Electrophotographic image forming apparatus and method Download PDF

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Publication number
US5250959A
US5250959A US07/698,103 US69810391A US5250959A US 5250959 A US5250959 A US 5250959A US 69810391 A US69810391 A US 69810391A US 5250959 A US5250959 A US 5250959A
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Prior art keywords
photosensitive member
image
toner
density
light
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US07/698,103
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English (en)
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Takanobu Yamada
Kazuyuki Fukui
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Minolta Co Ltd
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Minolta Co Ltd
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Assigned to MINOLTA CAMERA KABUSHIKI KAISHA reassignment MINOLTA CAMERA KABUSHIKI KAISHA ASSIGNMENT OF ASSIGNORS INTEREST. Assignors: FUKUI, KAZUYUKI, YAMADA, TAKANOBU
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/40—Picture signal circuits
    • H04N1/407—Control or modification of tonal gradation or of extreme levels, e.g. background level
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/23—Reproducing arrangements
    • H04N1/29—Reproducing arrangements involving production of an electrostatic intermediate picture
    • H04N1/295—Circuits or arrangements for the control thereof, e.g. using a programmed control device, according to a measured quantity

Definitions

  • the present invention relates generally to an electrophotographic image forming apparatus and method, and more particularly to an image forming apparatus and method of a type controlling an exposure output upon correction of gradations of image data.
  • image signals from an image reader such as a CCD or the like are initially converted to digital image data of, for example, eight bits (256 gradations) by an analog-to-digital converter (A/D converter).
  • A/D converter analog-to-digital converter
  • an exposure output is controlled upon correction of gradations of the digital image data based upon a gamma-correction table formulated in accordance with non-proportional gradation characteristics called the "gamma-characteristics" between the image data to be reproduced and reproduced image density levels.
  • the gamma-correction table is made in advance based upon a relationship between the image density and the image data and is stored in a ROM or the like.
  • any desired color reproduction needs the stability of the image density (the amount of adhesion of toner) of three different colors with respect to a photosensitive member.
  • an automatic toner density control (ATDC) and an automatic image density control (AIDC) are both performed in the apparatus. More specifically, in the ATDC, the toner density in a developer is detected by a toner density detector means and the amount of toner to be replenished is controlled in accordance with a detection signal thereof, thereby making the toner mixture ratio in the developer substantially constant.
  • the AIDC is performed to adjust the apparatus to changes in the amount of electrostatic charge of the developer or characteristic changes of the photosensitive member caused by environmental changes.
  • the density of a reference pattern formed on the photosensitive member is initially detected and developing conditions are changed in accordance with a detected density signal, thereby making the amount of adhesion of toner (the toner density) substantially constant.
  • a method of changing a developing bias is widely used with highest reliability as a method of changing the developing conditions.
  • the developing conditions are changed according to the environmental changes or the like, the gradation characteristics also change. Because of this, a single gamma-correction table cannot provide the optimum gradation correction, and therefore, the most appropriate gamma-correction table is selected from among a plurality of gamma-correction tables according to the developing conditions.
  • a central value in controlling the toner mixture ratio is occasionally changed.
  • the grid voltage and the developing bias are set low when the humidity is comparatively high.
  • the 256 gradations of the density must be controlled by a low developing voltage, and changes in surface potential or in the amount of light are followed by wide variations in the density.
  • the grid voltage and the developing bias are set high when the humidity is comparatively low. Since these values have respective upper limits due to the pressure resistance of the photosensitive member, saturation of the developing efficiency, or the like, the central value in controlling the toner mixture ratio is occasionally switched according to localities or seasons.
  • the image density is controlled to be constant by the AIDC.
  • desired images cannot always be obtained merely by selecting a gamma-correction table from among a plurality of tables in compliance with the AIDC.
  • the present invention has been developed to overcome the above-described disadvantages.
  • an electrophotographic image forming apparatus comprises a photosensitive member, a charging means for charging the photosensitive member with a given initial potential, an exposure means for forming an electrostatic latent image by applying light to the photosensitive member charged with the initial potential, an exposure control means for controlling the amount of light to be applied to the photosensitive member from the exposure means in compliance with an image density signal indicative of a density of each pixel of an image to be formed, and at least one developing device accommodating a developer consisting of toner and carrier.
  • the electrostatic latent image formed on the photosensitive member is developed by the developer contained in the developing device.
  • the apparatus further comprises a toner density detector means for detecting a toner density in the developer, a memory means for storing a plurality of gradation correction data, and a selector means for selecting one of the plurality of gradation correction data stored in the memory means based upon a toner density detected by the toner density detector means.
  • the exposure control means controls the amount of light based upon gradation correction data selected by the selector means.
  • an image forming method comprises the steps of:
  • an actual toner density is detected during an image forming operation, and an image formation is performed upon selection of appropriate gradation correction data corresponding to the actual toner density. Accordingly, even if the toner density changes, desired images can be stably obtained, of which gradations are appropriately corrected.
  • FIG. 1 is a schematic diagram of an image forming apparatus according to the present invention
  • FIG. 2 is a flow-chart indicative of a control required for an image forming operation according to a first embodiment of the present invention
  • FIG. 3 is a graph of a gamma-characteristic curve indicative of a relationship between input levels of image data and the image density
  • FIG. 4 is a graph of a gamma-correction characteristic curve indicative of a relationship between the input levels of image data and a laser output after gamma-correction;
  • FIG. 5 is a graph of several gamma-characteristic curves with the toner mixture ratio employed as a parameter
  • FIG. 6 is a graph of several gamma-correction characteristic curves corresponding to respective gamma-correction tables.
  • FIG. 7 is a flow-chart similar to FIG. 2, according to a second embodiment of the present invention.
  • FIG. 1 a full-color image forming apparatus embodying the present invention.
  • the image forming apparatus is internally provided with a photosensitive drum 1, a corona charger 2 having an arrangement called the "scorotron" for uniformly charging a photosensitive layer of the photosensitive drum 1, an exposure portion 3 for forming an electrostatic latent image upon exposure of an image formed, a plurality of developing devices 4a-4c for developing the electrostatic latent image using respective colored toners, and a transfer charger 5 for transferring a developed toner image to a transfer material 6.
  • the corona charger 2, the exposure portion 3, the developing devices 4a-4c, and the transfer charger 5 are located around the photosensitive drum 1 in this order in the direction of rotation of the photosensitive drum 1.
  • An AIDC sensor 7 for detecting the amount of toner adhering to the photosensitive drum 1 i.e., the amount of toner adhering to a reference pattern (this pattern is hereinafter referred to as an AIDC pattern) formed on the photosensitive drum 1 for detection of the image density is disposed downstream from the developing devices 4a-4c.
  • the developing devices 4a-4c are provided with respective ATDC sensors 8a-8c mounted thereon for detecting the toner mixture ratio in respective developers contained therein.
  • the developing devices 4a-4c accommodate toner replenishing means 9a-9c and developing sleeves 10a-10c, respectively.
  • the corona charger 2 has a grid 2a, of which the potential is controlled so as to be a predetermined potential corresponding to a surface potential of the photosensitive drum 1 by a grid voltage control circuit 11.
  • the exposure portion 3 receives laser light from a laser diode 12.
  • a developing bias is applied to the developing sleeves 10a-10c of the developing devices 4a-4c by a developing bias control circuit 13.
  • the toner replenishing means 9a-9c are controlled by a toner replenishment control circuit 14 so that the toner mixture ratio in respective developers may be made constant.
  • These grid voltage control circuit 11, developing bias control circuit 13 and toner replenishment control circuit 14 are controlled by a CPU 15.
  • the laser diode 12 is driven by a laser driver 18 on the basis of data obtained upon correction of image data from an image reader 16 such as a CCD or the like, using a gradation control circuit 17.
  • the gradation control circuit 17 is also controlled by the CPU 15.
  • Detection signals of the AIDC sensor 7 and those of the ATDC sensors 8a-8c are converted to digital signals by A/D converters 19a and 19b, respectively, and are inputted into the CPU 15.
  • the CPU 15 is electrically coupled with a ROM 20 in which are stored various control data including a plurality of gamma-correction tables and from which such data are outputted to the CPU 15 as occasion demands.
  • Step #1 Initialization is performed at step #1 followed by step #2 where various conditions for image formation such as, for example, the required number of copies are inputted.
  • Step #3 determines whether a print switch is data from the ATDC sensors 8a-8c and those from other sensors are inputted at step #4.
  • AIDC measurements are performed in which a grid voltage V G and a developing bias V B are set to respective predetermined reference values for the AIDC pattern and an output of the laser diode 12 is set to a predetermined value required for forming a half-tone AIDC pattern.
  • the corona charger 2 and a developing motor are activated and the developing bias V B is applied to the developing sleeves 10a-10c of the developing devices 4a-4c.
  • the AIDC pattern is then exposed by the laser diode 12 and is developed by the developing devices 4a-4c. Finally, the image density of the AIDC pattern is detected by the AIDC sensor 7.
  • step #6 based upon data outputted from the AIDC sensor 7, the most appropriate developing bias V B and grid voltage V G are selected using Table 1 below.
  • the data outputted from the AIDC sensor 7 are classified into density levels from 0 to 28.
  • Set values of the developing bias V B and the grid voltage V G are determined as corresponding to respective levels.
  • the density level detected by the AIDC sensor 7 increases with reduction of the AIDC pattern formed on the photosensitive drum 1 in density level. As a result, any image formation is performed with an optimum image density.
  • the selection of gamma-correction tables is performed at step #7.
  • Input levels of image data and the image density are in a non-proportional relationship called the "gamma-characteristics" as shown in FIG. 3. Accordingly, it is necessary to correct the output of the laser diode 12 with respect to the input levels of the image data, as shown in FIG. 4, in order to enhance the fidelity in an image formation of half-tone images. To this end, the correction-characteristics in the output of the laser diode 12 are tabulated.
  • Table 2 indicates an example of gamma-correction tables .sub. ⁇ 0,0 and .sub. ⁇ 1,0.
  • the gamma-characteristics change according to the toner density in a developer, as shown in FIG. 5, wherein the image density in a medium density portion becomes deep and that in a high density portion saturates. Accordingly, it is necessary to select one of the gamma-correction tables in compliance with the toner mixture ratio detected by the ATDC sensors 8a-8c.
  • gamma-correction tables are arrayed in the form of a matrix in accordance with detection levels of the AIDC sensor 7 and the toner mixture ratio and are stored in the ROM 20. From among these gamma-correction tables, the most appropriate one is selected on the basis of detection data of the AIDC sensor 7 and those of the ATDC sensors 8a-8c.
  • FIG. 6 indicates an example of the correction characteristics of gamma-correction tables .sub. ⁇ 0,0, .sub. ⁇ 0,1, .sub. ⁇ 0,2, .sub. ⁇ 0,3, .sub. ⁇ 0,4, when the detection level of the AIDC sensor 7 is "0".
  • step #8 an image forming operation is performed in accordance with image forming conditions set based upon detection values of the ATDC sensors 8a-8c, the CPU 15 gives the toner replenishment control circuit 14 instructions for replenishing toner by driving the toner replenishing means 9a-9c so that the toner mixture ratio in the developing devices 4a-4c may be a predetermined value.
  • step #9 determines whether the image forming operation is terminated. Unless the image forming operation is terminated, the procedure returns to step #6 to further proceed with an image forming operation in another color. In contrast, if the image forming operation is terminated at step #9, the procedure returns to step #2.
  • FIG. 2 may be replaced by a flow-chart shown in FIG. 7, which indicates a second embodiment of the present invention.
  • Steps from #11 to #13 in the flow-chart of FIG. 7 are the same as steps from #1 to #3 in the flow-chart of FIG. 2. In the flow-chart of FIG. 7, however, if a print switch is turned on at step #13, AIDC measurements and selection of the most appropriate developing bias V B and grid voltage V G are performed at step #14 and #15, respectively, prior to input of data from the ATDC sensors 8a-8c and other sensors.
  • detection values of the ATDC sensors 8a-8c are repeatedly inputted into. the CPU 15 at step #16 until an image forming operation is terminated.
  • the gamma-correction tables are selected based upon such detection values. Accordingly, even if the toner density is lowered during a large number of successive image forming operations, the gamma-correction tables are appropriately changed, thereby enabling the optimum half-tone image reproduction at all times.
  • the developing bias V B and the grid voltage V G are set on the basis of the detection signals of the AIDC sensor 7, the image formation is stably performed with an optimum image density. Furthermore, even if the toner density changes due to continuous image forming operations, the gradation characteristics can be maintained unchanged. The reason for this is that an output of the laser diode 12 is controlled in accordance with the most appropriate gamma-correction table selected from among a large number of gamma-correction tables on the basis of the detection signals of the AIDC sensor 7 and the ATDC sensors 8a-8c. As a result, half-tone images can be stably obtained.
  • the description is made with respect to an image forming apparatus of the intensity modulation type in which the laser driver 18 controls the intensity of light emitted from the laser diode 12 by changing an electric current supplied thereto on the basis of image data corrected by the gradation control circuit 17.
  • the present invention is also applicable to an image forming apparatus of the pulse-width modulation type in which the time period during which light is emitted from the laser diode 12 is controlled by changing the pulse-width of an electric current supplied to the laser diode 12 in compliance with image data.
  • the intensity of laser light from the laser diode 12 is controlled in compliance with input levels of image data and the gamma-correction tables are changed in compliance with detection values of the ATDC sensors 8a-8c in the toner density of developers.
  • an optimum dither matrix may be selected from among a plurality of threshold patterns (dither matrixes) stored in a ROM in compliance with detection values of the ATDC sensors 8a-8c.
  • the density of half-tone images can be controlled in compliance with changes in the toner density by controlling an output of the laser diode 12 using the selected dither matrix.
  • the present invention is also applicable to a laser printer, a facsimile machine, or the like.
  • a gradation correction table appropriate for a toner mixture ratio actually detected during an image forming operation is selectively determined. Accordingly, even if the toner mixture ratio changes, gradations of images are properly corrected, thus ensuring stable image formations.

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  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Control Or Security For Electrophotography (AREA)
  • Developing For Electrophotography (AREA)
  • Dry Development In Electrophotography (AREA)
  • Facsimile Image Signal Circuits (AREA)
  • Color Image Communication Systems (AREA)
US07/698,103 1990-05-15 1991-05-10 Electrophotographic image forming apparatus and method Expired - Lifetime US5250959A (en)

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JP2125823A JP2893861B2 (ja) 1990-05-15 1990-05-15 画像形成装置
JP2-125823 1990-05-15

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Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5453773A (en) * 1990-11-30 1995-09-26 Minolta Camera Kabushiki Kaisha Electrophotographic image forming apparatus comprising means for automatically adjusting image reproduction density
US5485191A (en) * 1991-09-18 1996-01-16 Canon Kabushiki Kaisha Image forming apparatus having tone correcting function
WO1996009717A1 (en) * 1994-09-19 1996-03-28 Apple Computer, Inc. Generation of tone reproduction curves using psychophysical data
US5512986A (en) * 1992-12-11 1996-04-30 Matsushita Electric Industrial Co., Ltd. Electrophotography apparatus
US5565995A (en) * 1989-10-31 1996-10-15 Canon Kabushiki Kaisha System for correcting a pulse-width-modulated signal in a recording apparatus
US5594533A (en) * 1994-06-30 1997-01-14 Kabushiki Kaisha Toshiba Image forming apparatus capable of varying charge amount in accordance with toner density
US5694223A (en) * 1995-03-07 1997-12-02 Minolta Co., Ltd. Digital image forming apparatus which specifies a sensitivity characteristic of a photoconductor
US5805314A (en) * 1994-07-22 1998-09-08 Ricoh Company, Ltd. Color image forming apparatus separately correcting each color component image
US6154288A (en) * 1996-11-08 2000-11-28 Fuji Photo Film Co. Ltd. Printer and image processing method for adjusting printing criteria to compensate for low/high density images
US6501492B1 (en) * 1992-11-13 2002-12-31 Canon Kabushiki Kaisha Image forming apparatus for outputting image in response to digital image signal
US20040156116A1 (en) * 2003-01-17 2004-08-12 Pentax Corporation Light-projecting device
US20040156629A1 (en) * 2003-01-17 2004-08-12 Pentax Corporation Light-projecting device
US6879781B2 (en) 2003-01-17 2005-04-12 Pentax Corporation Light-projecting device

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR20060019140A (ko) 2004-08-26 2006-03-03 삼성전자주식회사 바코드 스캐너를 구비하는 전자레인지

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JPS5147435A (ja) * 1974-10-21 1976-04-23 Ricoh Kk
US4277162A (en) * 1978-07-13 1981-07-07 Ricoh Company, Ltd. Electrophotographic apparatus comprising density sensor means
US4801980A (en) * 1986-10-29 1989-01-31 Konica Corporation Toner density control apparatus
JPH01196347A (ja) * 1988-02-02 1989-08-08 Canon Inc 画像形成装置
US4873428A (en) * 1987-03-03 1989-10-10 Canon Kabushiki Kaisha Image processing apparatus using conversion means
US4914459A (en) * 1988-04-18 1990-04-03 Ricoh Company, Ltd. Image forming apparatus

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5147435A (ja) * 1974-10-21 1976-04-23 Ricoh Kk
US4277162A (en) * 1978-07-13 1981-07-07 Ricoh Company, Ltd. Electrophotographic apparatus comprising density sensor means
US4801980A (en) * 1986-10-29 1989-01-31 Konica Corporation Toner density control apparatus
US4873428A (en) * 1987-03-03 1989-10-10 Canon Kabushiki Kaisha Image processing apparatus using conversion means
JPH01196347A (ja) * 1988-02-02 1989-08-08 Canon Inc 画像形成装置
US4914459A (en) * 1988-04-18 1990-04-03 Ricoh Company, Ltd. Image forming apparatus

Cited By (16)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5565995A (en) * 1989-10-31 1996-10-15 Canon Kabushiki Kaisha System for correcting a pulse-width-modulated signal in a recording apparatus
US5453773A (en) * 1990-11-30 1995-09-26 Minolta Camera Kabushiki Kaisha Electrophotographic image forming apparatus comprising means for automatically adjusting image reproduction density
US5485191A (en) * 1991-09-18 1996-01-16 Canon Kabushiki Kaisha Image forming apparatus having tone correcting function
US6501492B1 (en) * 1992-11-13 2002-12-31 Canon Kabushiki Kaisha Image forming apparatus for outputting image in response to digital image signal
US5512986A (en) * 1992-12-11 1996-04-30 Matsushita Electric Industrial Co., Ltd. Electrophotography apparatus
US5594533A (en) * 1994-06-30 1997-01-14 Kabushiki Kaisha Toshiba Image forming apparatus capable of varying charge amount in accordance with toner density
US5805314A (en) * 1994-07-22 1998-09-08 Ricoh Company, Ltd. Color image forming apparatus separately correcting each color component image
WO1996009717A1 (en) * 1994-09-19 1996-03-28 Apple Computer, Inc. Generation of tone reproduction curves using psychophysical data
US6345128B1 (en) 1994-09-19 2002-02-05 Apple Computer, Inc. Generation of tone reproduction curves using psychophysical data
US5694223A (en) * 1995-03-07 1997-12-02 Minolta Co., Ltd. Digital image forming apparatus which specifies a sensitivity characteristic of a photoconductor
US6154288A (en) * 1996-11-08 2000-11-28 Fuji Photo Film Co. Ltd. Printer and image processing method for adjusting printing criteria to compensate for low/high density images
US20040156116A1 (en) * 2003-01-17 2004-08-12 Pentax Corporation Light-projecting device
US20040156629A1 (en) * 2003-01-17 2004-08-12 Pentax Corporation Light-projecting device
US6879781B2 (en) 2003-01-17 2005-04-12 Pentax Corporation Light-projecting device
US6903885B2 (en) 2003-01-17 2005-06-07 Pentax Corporation Light-projecting device
US6941069B2 (en) 2003-01-17 2005-09-06 Pentax Corporation Light-projecting device

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JP2893861B2 (ja) 1999-05-24

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